This space invention relates to position transducing.
In many applications it is important to measure and to control the angular position of a rotary element with high accuracy. For instance, galvanometers require sensors which must detect the angular position of the rotor with extreme accuracy and repeatability, high temperature stability, and high signal to noise ratio. To enable higher speed of operation, in addition, these sensors must be small in size and have low inertia. To meet the requirement of very accurate detection at low cost has, in the past, required undesirable compromises.
In the field of galvanometers the precision requirements for the angular position detection may be one part in one thousand or even one part in one million, depending upon the application. Detection at this accuracy has, for example, been achieved using variable-inductance transducers or variable-capacitance transducers. The variable-inductance transducers utilize changes in the mutual inductance dependent on the angular position of the rotor. The variable-capacitance transducers employ changes in capacitance caused by the angular motion of the rotor. Usually, a stationary portion of the variable-capacitance transducer is formed by a pair of capacitor plates, held as one frame of reference, and a moving portion is a rotating dielectric located between the capacitor plates, which is held as another frame of reference. Both types of transducers are used in a closed-loop system or in an open-loop system. A closed-looped system is designed to precisely position rotatable elements since it can compensate for errors due to external forces or nonlinearities. When this system is calibrated these errors are measured and a position feedback mechanism is used to improve the accuracy of the device. If a system alternatively uses an open-loop arrangement, such system can perform the above mentioned corrections and can also report the orientation of the rotor. The present transducer replaces the variable inductance transducer or the variable-capacitance transducer in such systems.
Angular position transducers are used not only to detect the angular position of the rotatable element but also to control and hold the rotatable element in a desired position. For example, a galvanometer can be used to drive a mirror. The mirror is used to deflect a laser beam which in turn modifies material or objects as in microlithography, reprographic applications, exposure of photosensitive films or papers, micromachining, laser trimming of film resistors, or repairing of memory circuits, for entertainment or metrology such as laser scanning microscopes. The present invention is also useful in these cases.